It is learned that the demand for lower carbon olefins is increasing, but the starting material resources, e.g. naphtha, light diesel oil and so on, for the production of ethylene is confronted with serious shortage. The methanol to olefin (hereinafter referred to as MTO) or dimethyl ether to olefin (hereinafter referred to as DTO) technique, therefore, has drawn much attention. MTO or DTO technique is a technology for producing lower carbon olefins, such as ethylene, propylene and the like, by using coal-based or natural gas-based synthesized methanol or dimethyl ether as the starting materials and by means of a fluidized bed reaction form similar to a catalytically cracking device. Said technology can be used in the production of lower carbon olefins with a high selectivity, and the propylene/ethylene ratio may be optionally adjusted within a relatively broad scope.
Generally, the product gas obtained by the catalytic reaction of methanol via a MTO reaction device contains hydrogen, methane, ethylene, ethane, propylene, propane, and higher carbon olefins, such as butylene or pentene, as well as water, carbon monoxide and carbon dioxide. Except that the conditions for DTO production are readily controllable, DTO technique is substantially the same as MTO technique, so that the DTO reaction product is also substantially the same as MTO reaction product. In order to obtain a polymerizable grade of ethylene product, the separation and recovering techniques for MTO or DTO product gas are very essential. Moreover, U.S. Pat. No. 5,811,621 has depicted such a technique for recovering ethylene. Generally, said technique for recovering ethylene comprises subjecting MTO product gas to the conventional gas liquid separation step to separate and remove water, carbon dioxide and heavy components of C5 and higher; introducing C4 and light components after cooling into the deethanizing column; discharging ethane and light components from the top of the deethanizing column; and then feeding them into the demethanizing column after removing ethyne via a hydrogenation device; removing methane and hydrogen from the top of the demethanizing column, to introduce the materials at the column bottom substantially containing ethylene and ethane into the C2 component separation column; and obtaining ethylene product at the top of the C2 component separation column. In said technology for recovering ethylene, many compression and pressure-increasing processes are required. Generally, the pressure of C4 and light components is required to be increased to 20-30 atm·A before being fed into the deethanizing column. In addition, another pressure-increasing step is further needed to increase the pressure to higher than 30 atm·A after removing ethyne via the hydrogenation reaction device and before feeding into the demethanizing column. In order to increase the recovery rate of ethylene, the materials at the top of the demethanizing column is further compressed and pressure-increased, and the compressed gas provides heat for the reboiler of the demethanizing column, and then is partially condensed. The uncondensed gas contains ethylene in an amount of 3-4% of the feedstock, and is discharged. It can be seen that three processing compressors and a propylene refrigeration compressor are used in said ethylene recovering technique, and thus said technique needs a high equipment investment. Meanwhile, ethylene-containing uncondensed gas is discharged from said technique. Accordingly, the total ethylene recovery rate, generally about 95-97%, is not satisfactory and does not satisfy the standard of the polymerizable grade of ethylene.
Accordingly, it still needs a process for recovering lower carbon olefins from a product gas from the methanol to olefin device and/or dimethyl ether to olefin device. Said process not only saves energy, but also can be used for recovering lower carbon olefins with a high yield, so as to obtain the polymerizable grade of ethylene and propylene products.